This invention relates to devices for collecting particles such as cells suspended in liquids, and solutes carried in solvents. More particularly, the invention relates to filter devices for distributing particles or solutes in a graduated fashion to allow enumeration over a wide range of particle or solute concentrations.
Enumeration of microorganisms in water, foods, pharmaceuticals, etc., is commonly done by the application of samples to petri plates containing mixtures of nutrients and agar. Individual cells multiply on the surface of the agar to form visible colonies which are then counted. Since the number of organisms in the sample is unknown, a series of dilutions is usually prepared and aliquots (usually 0.1 mL) of each dilution are spread on the agar surface. The number of colonies per standard (10 cm) plate should be above 30 (for statistical significance) and below 300 (above which point overlapping colonies become more likely), unless more elaborate methods are used (Arthur L. Koch in Manual of Methods for General Bacteriology, Philipp Gerhardt, ed., American Society for Microbiology, 1981, p. 185).
An alternative known method for determining bacterial counts is to pass the sample through a membrane filter which retains the bacteria on its surface. The filter is then placed on the surface of an agar plate. Nutrients diffuse through the membrane allowing cells to grow into visible colonies. Advantages of membrane filtration are that (i) large volumes can be filtered to enumerate bacteria in dilute samples; and (ii) distribution of cells over the filter is uniform, a result that is difficult to achieve with spreading methods. Accurate quantitation, however, often requires multiple dilutions and filtrations, which are laborious and expensive.